Skip CI builds for PRs that only touch build/test-irrelevant files (#369)

Docs, GitHub Actions config, the legacy Makefile build, unreachable
demo apps, and dev-only tooling scripts can't affect the outcome of
this buildbot's CMake configure/build/ctest pipeline, so PRs touching
only those paths no longer schedule a build. Also authenticates the
GitHub PR-files API lookup (via the existing GITHUB_TOKEN) to reduce
the chance of a rate-limited/truncated file list; the filter fails
open (never skips) when the file list is empty.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: alexreinking

master/master.cfg

@@ -2,6 +2,7 @@
 # ex: set syntax=python:
 # vim: set syntax=python:
 # comment
+import fnmatch
 import logging
 import operator
 import os
@@ -1078,13 +1079,86 @@ def create_builders():
 
 c["builders"] = list(create_builders())
 
+
+# Paths that cannot affect the outcome of this buildbot's CMake configure/build/ctest
+# pipeline (it never invokes the legacy Makefile, builds docs, runs pip packaging, or
+# runs clang-tidy/pre-commit). fnmatch's `*` already matches across `/`, so these behave
+# like `**` globs without needing separate syntax.
+_IGNORABLE_PATH_GLOBS = [
+    # GitHub Actions / GitHub metadata -- this buildbot doesn't use GH Actions at all
+    ".github/*",
+    # Root-level and nested docs
+    "*.md",
+    "doc/*",  # Doxygen docs; WITH_DOCS is OFF by default
+    "LICENSE.txt",
+    # Repo/editor/tooling metadata that never reaches CMake or ctest
+    ".gitignore",
+    ".gitattributes",
+    ".gitmodules",
+    ".gdbinit",
+    ".lldbinit",
+    ".clang-format",
+    ".clang-format-ignore",
+    ".clang-tidy",
+    ".git-blame-ignore-revs",
+    ".devcontainer/*",
+    ".claude/*",
+    ".idea/*",
+    # Coverage / pre-commit / lint configs and scripts (not run by this buildbot)
+    "codecov.yml",
+    ".pre-commit-config.yaml",
+    "run-clang-format.sh",
+    "run-clang-tidy.sh",
+    # pip packaging -- buildbot builds python bindings via CMake+uv directly,
+    # never via packaging/pip's sdist/wheel path
+    "packaging/*",
+    # Legacy `make`-based build -- this buildbot only builds via CMake
+    "Makefile",
+    "Makefile.inc",
+    "*/Makefile",
+    "*/Makefile.inc",
+    "apps/support/*",  # legacy-make helpers only (Makefile.inc, viz_auto.sh)
+    # Apps that apps/CMakeLists.txt never builds
+    "apps/HelloAndroid/*",
+    "apps/HelloAndroidCamera2/*",
+    "apps/HelloiOS/*",
+    "apps/HelloWasm/*",
+    # Dev-only tools/ scripts not wired into tools/CMakeLists.txt or any test target
+    "tools/find_inverse.cpp",
+    "tools/check_cmake_file_lists.py",
+    "tools/check_cmake_style.py",
+    "tools/clang-tidy-filter.sh",
+    "tools/gdbhalide.py",
+    "tools/lldbhalide.py",
+    "tools/Halide.natvis",
+    "tools/halide_config.make.tpl",
+    "tools/makelib.sh",
+    "tools/run-coverage.sh",
+]
+
+
+def _is_ignorable_path(path):
+    return any(fnmatch.fnmatchcase(path, pattern) for pattern in _IGNORABLE_PATH_GLOBS)
+
+
+def is_change_important(change):
+    files = change.files
+    if not files:
+        # Fail open: an empty/missing file list (e.g. a rate-limited or
+        # unauthenticated GitHub API call) must never cause a build to be skipped.
+        return True
+    return any(not _is_ignorable_path(f) for f in files)
+
+
 c["schedulers"] = [
     AnyBranchScheduler(
         name="halide-main",
         change_filter=ChangeFilter(
             category="pull",
             branch_fn=lambda br: br != "main",
         ),
+        fileIsImportant=is_change_important,
+        onlyImportant=True,
         builderNames=[b1.name for b1 in c["builders"]],
     ),
     ForceScheduler(
@@ -1247,6 +1321,7 @@ c["www"] = {
             "secret": WEBHOOK_TOKEN,
             "skips": [],
             "class": SafeGitHubEventHandler,
+            "token": GITHUB_TOKEN,
         },
     },
 }